Basin Hydrodynamic Considerations in Planning and Drilling Exploration Wells in the Deepwater

By

Saad T. Saleh¹

(1) Knowledge Systems, Inc, Stafford, TX

 Planning and construction of deepwater wells require accurate pre-drill geopressure prediction as well as real-time monitoring. One of the most critical elements of the pre-drill geopressure estimation is the recognition of hydrodynamic effects in fluid-filled sand bodies that will be penetrated by the planned well. Depending on the structural position of the well penetration into sand structures, the pressure in the sand can be significantly higher or lower than the bounding shale. Failure to account for and anticipate such hydrodynamic effects (typically called the Centroid effect in recent literature) may lead to an incorrect well plan, and ultimately, to significant drilling problems, which may force premature well abandonment (not reaching target).

Conventional geopressure estimation methods are based on estimating pore pressure from compaction-dependent geophysical properties. Compaction is lithology-dependent and is greatest in shales. The assumption of pressure equilibrium between sand and shale is not reliable in many cases. Pore pressure anomalies between sand and shale should be investigated. One of the causes of pressure differences is the updip pressure transfer (Centroid effect). We present a methodology for recognizing the Centroid effect from offset well data. In addition, we discuss the effect of well position and other factors that affect the estimated pore pressure profile in shale and sand bodies. The intricate complexity of the physical and chemical systems involved in the evolution of the Centroid effect is demonstrated in several real world examples.